Bungie have announced preload details for the upcoming Destiny 2 New Light and one important note - all the existing players will need to uninstall their current game and download the new one in order to play it.
Destiny 2 preloading is already live on Steam so PC players can get their copy of New Light roughly a week before the official launch. This is extremely convenient to new players but the transition has an unwanted side effect to those who have the Battle.net version of Destiny 2. These players will need to download the Steam version in order to play from 1 October 2019 onwards.
Therefore, PC players will need to keep two copies of Destiny 2 on their storage disks until the launch of Shadowkeep when it will be safe to delete the one from Blizzard's launcher.
New Light will have a somewhat similar effect on console players as well since they will need to preload Shadowkeep too. Unfortunately, their situation is a bit worse than that of PC players. Preloading is not available for console at the moment as players will need to keep playing the current version of Destiny 2 until 30 September 2019.
This is when 24-hour maintenance will kick in and all the servers will go down for the duration. It is these 24 hours the console players will have as a preload period Destiny 2 before Shadowkeep launch. At least there are no notions they will need to redownload the entire game for the moment.
Following the maintenance and preload periods, players will be able to play the new content at the very start of October 2019. PC players who didn't purchase Forsaken will be able to do so on the same date, which is not really a player-friendly move from Bungie as many newcomers could be misled into believing they would get all the content with Shadowkeep purchase.